In a column published in the Financial Times, he argued that the UK's looming exit makes the Nato alliance a more important force for co-operation in the region.

"I believe that our nations must summon the political will, and make concrete commitments, to meet these urgent challenges. I believe we can - but only if we stand united as true allies and partners," he wrote.

The president's words were published as he opened two days of meetings with European Union and Nato leaders in Warsaw.

Mr Obama began his day by meeting European Council president Donald Tusk and European Commission president Jean-Claude Juncker. The White House said he was urging the leaders to step carefully in the exit negotiations, which have not yet been formally triggered by Britain and could take up to two years.

"I am confident that the UK and the EU will be able to agree on an orderly transition to a new relationship, as all our countries stay focused on ensuring financial stability and growing the global economy," Mr Obama wrote.

Mr Obama's trip, which includes a stop in Spain, is expected to be his last trip to Europe as president.

The task of trying to shape the talks to serve US interests and mitigate damage will largely fall to his successor, but in his remaining time in office, he has sought to use his popularity in Europe and his presidential megaphone to defend international co-operation and the "European project" and will urge other leaders to speak up more forcefully.

The White House has acknowledged that Mr Obama's message has to some degree failed to persuade on both sides of the Atlantic. Republican presidential candidate Donald Trump has suggested he would seek to pull back from Europe, even hinting the US could withdraw from Nato, the 67-year-old cornerstone of European security. His Democratic rival, Hillary Clinton, has suggested she would continue, if not deepen, Mr Obama's approach, but she has rejected the president's push for massive multinational free-trade agreements.

Nato secretary-general Jens Stoltenberg said Moscow's hostile actions in Ukraine have spurred the alliance to raise its defences on the eastern flank.

He spoke to reporters before the Nato summit opened to approve, among others, the presence of four battalions in Poland and the Baltic states. These nations feel threatened after Russia annexed Crimea from Ukraine in 2014 and continues to back separatists in eastern Ukraine.

Mr Stoltenbeg said: "No-one was talking about reinforcing deterrence" before the action in Crimea.

He added that deterrence and defence combined with constructive dialogue are the best approach in ties with Russia, adding that the Russia-Nato Council will meet next week.

A spokesman for Russian president Vladimir Putin said Moscow is willing to co-operate with Nato although it acts towards Russia like an enemy.

Dmitry Peskov said Moscow "has always been open for dialogue" with Nato, especially to fight what it sees as a "genuine threat" - terrorism.

Mr Peskov added: "Russia is not looking (for an enemy) but it actually sees it happening. When Nato soldiers march along our border and Nato jets fly by, it's not us who is moving closer to the Nato borders."